We have determined that the mammalian uridine diphospho-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase T1 (EC 2.4.1.41) has the appropriate acceptor substrate specificity to recognize the non-glycosylated form of contulakin-G (ZSEEGGSNATKKPYIL-OH where Z=pyroglutamic acid) and to transfer GalNAc to the peptide. Both [Thr(10)] contulakin-G and a pre-contulakin-G(30-66) (RGLVPDDITPQLILGSLISRRQSEEGGSNATKKPYIL-OH) were shown to be acceptors for the mammalian enzyme. The site of attachment of the GalNAc residue was determined using chemical and radioactive sequencing techniques. The mammalian enzyme was highly specific for Thr(10) residue, in which the native peptide was found to be glycosylated, compared with either Ser(2) or Ser(7). In the case of pre-contulakin-G, the enzyme was also highly specific for the equivalent threonine residue. These results suggest that the Cone snail uses an enzyme with similar acceptor specificity to that of the mammalian polypeptide N-acetylgalactosaminyltransferase for glycosylating contulakin-G.